Method of soldering with phosphoric acid soldering flux

ABSTRACT

A phosphoric acid flux with good fluxing efficiency and high viscosity, and one which is especially suitable for use in the soldering of copper and copper-plated stainless steel. The flux is in the form of a viscous fluid, or a thermally unstable emulsion when the fluid is combined with a pasty vehicle. The viscous fluid contains phosphoric acid and an organic material (molecular weight between 4,000 and 20,000) from the group consisting of polyethers and polyimines or mixtures thereof having an atomic ratio of carbon to ether oxygen plus imine nitrogen between 1.5 and 3.1.

This a division, of application Ser. No. 535,744, filed Dec. 23, 1974,now U.S. Pat. No. 3,985,586, issued Oct. 12, 1976.

The present invention relates to a soldering flux and, moreparticularly, to a soldering flux especially suitable for use in thesoldering of copper and copper-plated stainless steel.

Although phosphoric acid is an excellent soldering flux for copper andcopper-plated stainless steel, it has not been generally accepted by theplumbing trade because of its low viscosity. Attempts to overcome theviscosity problem by blending petrolatum and other pasty vehicles withthe phosphoric acid failed to produce a suitable flux. The pastyvehicles improved texture but reduced the fluxing efficiency of theflux. A further attempt to produce a suitable flux by blending lesspetrolatum and more phosphoric acid also proved ineffectual. Theresulting flux was an unstable emulsion, one in which the acid separatedfrom the petrolatum.

The present invention overcomes the heretofore referred to shortcomingof phosphoric acid fluxes by providing one with good fluxing efficiencyand high viscosity. To phosphoric acid is added an agent which issoluble or miscible therewith, and one which increases its viscosity atboth room and soldering temperatures. The agent is an organic material(molecular weight between 4,000 and 20,000) from the group consisting ofpolyethers and polyimines or mixtures thereof having an atomic ratio ofcarbon to ether oxygen plus imine nitrogen between 1.5 and 3.1.

It is accordingly an object of the present invention to provide aphosphoric acid flux having good fluxing efficiency and high viscosity.

The flux of this invention can take the form of a viscous fluid or athermally unstable emulsion. The emulsion results when the viscous fluidis combined with a pasty vehicle to form a mixture having from 30 to 70%by weight, of pasty vehicle. In weight percent, the viscous fluidconsists essentially of 35 to 85% phosphoric acid (based on aconcentration of 75 to 95%), 15 to 65% of organic material from thegroup consisting of polyethers and polyimines or mixtures thereof havingan atomic ratio of carbon to ether oxygen plus imine nitrogen between1.5 and 3.1, and from 0 to 30% water. The molecular weight of theorganic material is between 4,000 and 20,000. For purposes ofdefinition, ether or imine functionalities which are present as branchesto the main polymer chain (as the methoxyl groups in a methylatedglucoside) are counted. All of the carbon atoms in the molecule arecounted, but oxygen or nitrogen which is not present as ether or iminelinkages is not counted.

Phosphoric acid is present in the viscous fluid in an amount of from 35to 85% and preferably 40 to 80%, based upon the use of 75 to 95%phosphoric acid. If the concentration is too low, the resulting mixturewill be either a solid or a gum, and hence not conducive for applicationto the surface to be soldered. Too high a concentration of phosphoricacid results in a mixture which is too fluid to remain in the joint atsoldering temperature.

The molecular weight of the organic material is between 4,000 and20,000, and preferably between 6,000 and 10,000. Organic materials withtoo low a molecular weight are not capable of providing the fluid withsufficient viscosity. On the other hand, organic materials with too higha molecular weight will yield a flux with too high a viscosity toadequately flow. Typical organic materials which satisfy the criteria ofthe present invention are polyethylene glycol, polypropylene glycol,methyl cellulose and polyethyleneimine. Depending upon the molecularweight of the organic material various amounts of water can be added tothe flux. For higher molecular weight organic materials, we have reasonto believe that up to 30% water can be added. Water levels will,however, generally be below 20%. The preferred level of organic materialis from 20 to 60%.

A pasty vehicle can be added to the viscous fluid to turn it into athermally unstable emulsion. Users of the flux usually find the textureof the emulsion more desirable to work with. At soldering temperatures,the emulsion breaks and separates into components. Typical pastyvehicles are petrolatum, animal fats and hydrogenated vegetable and nutoils. As a general rule the mixture should have at least 30%, by weight,of a pasty vehicle. Levels in excess of 70% can result in too stable anemulsion and poor wetting of the surface to be soldered. A preferredrange is from 33 to 40%. The emulsified flux of this invention canaccomodate greater percentages of phosphoric acid than could previouslyknown phosphoric acidpasty vehicle fluxes, as the organic materialdescribed hereinabove is included therein.

The present invention also envisions additions of wetting agents, dyes,and oxidizing agents such as ammonium nitrate. Such additions are not,however, necessary for the attainment of the desired properties.

The following examples are illustrative of several aspects of theinvention. As stated hereinabove the soldering flux described herein isespecially suitable for use in the soldering of copper and copper-platedstainless steel.

EXAMPLE I

The following were mixed together in order to form a flux:

37.5gm Polyethylene glycol (MW-6000)

25.0gm Phosphoric acid (85%)

37.5gm Petrolatum

The mixture contained 62.5% viscous fluid and 37.5% pasty vehicle. Itwas heated to 150° F to melt the polyethylene glycol and the petrolatumand stirred vigorously while it cooled to 120° F. From 120° F it wasallowed to cool without stirring.

Sections of 3/4 inch diameter T437 stainless steel tubing bearing acopper plating on the exterior surface were joined using copper sleevefittings and 50 Pb/50 Sn solder. The assembled joints were held in a jigin a fixed reproducible relationship to an air-acetylene flame. Thejoints were fluxed and heated for a measured time; then the flame wasremoved and solder was fed thereto. One of the joints was prepared usingthe above described flux. It was heated for 30 seconds and a water-tightjoint was produced. Under the same conditions, a flux prepared from 25%phosphoric acid (85%) and 75% petrolatum failed when the heating timeexceeded 17 seconds. Note the advantage of replacing some of thepetrolatum with polyethylene glycol. The flux with polyethylene glycolwas stored for two months without any separation of the emulsionoccurring.

EXAMPLE II

The following were mixed together in order to form a flux:

58.8 gm Phosphoric acid (75%)

41.2 gm Polyethylene glycol (MW-6000)

The mixture was heated to 175° F to melt the polyethylene glycol. Oncethe polyethylene glycol was melted, the mixture was stirred to make ithomogeneous and allowed to cool. Joints prepared in the manner of theprevious example, but with the liquid flux of this example, were foundto be water-tight even when the heating time was 55 seconds.

It will be apparent to those skilled in the art that the novelprinciples of the invention disclosed herein in connection with specificexamples thereof will suggest various other modifications andapplications of the same. It is accordingly desired that in construingthe breadth of the appended claims they shall not be limited to thespecific examples of the invention described herein.

I claim:
 1. In the soldering of copper and copper-plated stainless steel wherein metal selected from the group consisting of copper and copper-plated stainless steel is placed in close proximity with other metal, solder and soldering flux, and wherein said solder is melted and applied to said metals in such a way that it solidifies and effects a bond therebetween; the improvement which comprises utilizing a soldering flux comprised of a viscous fluid; said fluid consisting essentially of, in weight percent, 35 to 85% phosphoric acid, 15 to 65% of organic material selected from the group consisting of polyethers and polyimines and mixtures thereof having an atomic ratio of carbon to ether oxygen plus imine nitrogen between 1.5 and 3.1, and from 0 to 30% water; and organic material having a molecular weight between 4,000 and 20,000.
 2. An improvement according to claim 1, wherein said viscous fluid has 40 to 80% phosphoric acid, 20 to 60% organic material, and 0 to 20% water.
 3. An improvement according to claim 1, wherein said organic material has a molecular weight between 6,000 and 10,000.
 4. An improvement according to claim 1, wherein said organic material is from the group consisting of polyethylene glycol, polypropylene glycol, methyl cellulose and polyethyleneimine.
 5. An improvement according to claim 4, wherein said organic material is polyethylene glycol.
 6. An improvement according to claim 1, wherein said flux consists essentially of said viscous fluid and from 30 to 70%, by weight, of pasty vechicle.
 7. An improvement according to claim 6, wherein said pasty vehicle is petrolatum.
 8. An improvement according to claim 6 wherein said flux has from 33 to 40% of said pasty vehicle. 